FLOTATION
INTRODUCTION

Flotation is an operation that removes not
only oil and grease but also suspended
solids from wastewaters.

The waste flow or a portion of clarified
effluent is pressurized in the presence of
sufficient air to approach saturation.
Waste flow is pressurized to approach saturation
released to the atmospheric pressure
Minute air bubbles are released from the solution
SS, oil & grease, sludge flocs are floated
attachment with air bubbles
Enmeshed in the floc particles
Air-solids mixture rises to the surface
AIR SOLUBILITY AND RELEASE

The saturation of air in water is directly
proportional to pressure and inversely
proportional to temperature.

The quantity of air that will theoretically
be released from solution when the
pressure is reduced to 1 atm:
AIR SOLUBILITY AND RELEASE
AIR SOLUBILITY AND RELEASE

The actual quantity of air released will
depend
(i) upon the turbulent mixing conditions at
the point pressure reduction
(ii) on the degree of saturation obtained in
the pressurizing system
AIR SOLUBILITY AND RELEASE

Since the solubility in industrial wastes may be
less than that in water, a correction have to be
applied :
FLOTATION UNIT

The performance of a flotation system
depends upon having sufficient air bubbles
present to float substantially all of the
suspended solids .

An insufficient quantity of air will result in
only partial flotation of the solids, and
excesive air will yield no improvement.
FLOTATION UNIT

The performance of a flotation unit terms of
effluent quality and solids concentration in the
float can be related to an air/solids ratio:
FLOTATION UNIT
FLOTATION UNIT

When pressurized recycle is used:
DAF

The most common procedure is that of dissolved
air flotation (DAF), in which the waste stream is
first pressurized with air in a closed tank.

After passing through a pressure-reduction
valve, the wastewater enters the flotation tank
where, due to the sudden reduction in pressure,
minute air bubbles in the order of 50- 100
microns in diameter are formed.
DAF
As the bubbles rise to the surface, the
suspended solids and oil or grease
particles adhere to them and are carried
upwards.
 It is common practice to use chemicals to
enhance flotation performance.
 One alternate design involves the
recycling of part (10-30%) of the treated
water.

DAF
All systems contain a mechanism for
removing the solids that may settle to the
bottom of the flotation tanks, usually by a
helical conveyor placed in the conical
bottom.
 The main advantage claimed of DAF
systems is the faster rate at which very
small or light suspended solids can be
removed in comparison with settling.

DAF
Performance of DAF systems has been reported
to be dependent on several factors:
the solids concentration
the ratio of air to solids (A/S)
 Key factors in the successful operation of DAF
units are
the maintenance of proper pH
proper flow rates
the continuous presence of trained operators

DAF
DAF

Diagram of a DAF system with recycle
DAF
DAF
DAF
SURFACE AERATOR
Another flotation system exists in which
air is not dissolved but forced through the
wastewater by surface aerators.
 This system generates air bubbles of
larger size than DAF systems.
